The present invention relates to a printing assembly. More particularly this invention concerns a rotary press for offset printing on both sides of a web or sheet.
It is known to pass a web or sheet between a pair of blanket cylinders each tangentially engaging a respective plate cylinder so as simultaneously to print on both sides of the sheet or web. Moistening and/or inking rollers are associated in turn with each of the plate cylinders.
It is essential in such an arrangement that the various cylinders be exactly aligned with each other. To this end they are always mounted in a frame constituted by a pair of walls or supports. Each cylinder has two end pins which are each journalled in a respective one of the two spaced-apart supports. Adjustment means of some type must be provided on each of the supports so as to displace the journal axes of the cylinders for a proper alignment of the cylinders with one another.
Typically the cylinders are mounted between the two supports by first inserting the pin or peg at one end of each of the cylinders through one of the supports and pushing it therethrough far enough so that the other end peg can be aligned with the appropriate journal in the other support whereupon the cylinder is pulled back and also inserted in the other support.
Such an arrangement, therefore, requires that at least one of the journal pegs on each of the cylinders be relatively long so as to permit the axial shifting of the cylinder for mounting between the supports. This increases the fabrication cost of the cylinder. Another disadvantage of this arrangement is that it is therefore necessary that the cylinder be spaced at least at one of its ends by a distance at least equal to the depth with which the other end is inserted in the respective journal. Thus the cylinders are supported relatively far from their centers, thereby increasing the possibility of bending of these cylinders while at the same time making control of them and adjustment substantially more difficult.
It has been suggested to overcome this difficulty in part by means of complicated mounting arrangements which often are little better than the systems they replace, and which greatly increase the cost of the printing assembly. Another suggestion has been to increase the roller diameters so that, even though they are parted relatively widely, the rollers are unlikely to bend. This increased size once again increases the bulk and cost of the printing assembly.
It is also known, as for instance disclosed in British Pat. No. 843,899, to mount the two plate cylinders one above the other in a common vertical plane and to provide the respective blanket cylinders between them with the axes of these blanket cylinders flanking the plane of the axes of the plate cylinders. Such an arrangement requires relatively complex and expensive adjustment gear to allow the various cylinders to be exactly aligned with one another. It is essential in a printing operation that each of the blanket cylinders engage its respective plate cylinder along the full length thereof with enough force to transfer the ink, but not so strongly as to damage the plate cylinder. Furthermore each of the blanket cylinders must engage the paper with sufficient force and evenness to transfer the ink to the paper, but not so strongly as to damage the paper or blur the image.